In recent years, connecting parts such as automotive terminals and connectors are required to have a capability of securing reliability under high-temperature environments like in engine rooms. For the reliability under high-temperature environments, one of the most important properties is maintenance properties of mating force at a contact point, so-called, resistance property of stress relaxation.
FIG. 2 shows a structure of a box connector (female terminal 3) that is typical as a connecting part of automotive terminals, connectors or the like. FIG. 2(a) shows a front view, and FIG. 2(b) shows a cross-sectional view. In FIG. 2, the female terminal 3 is so designed that the push part 5 is semi-supported by the upper holder section 4. When the male terminal (tab) 6 is inserted into the holder, then the push part 5 undergoes elastic deformation, and owing to the reaction force thereto, the male terminal (tab) 6 is thereby fixed. In FIG. 2, 7 is a wire connecting part, and 8 is a fixing segment.
As in FIG. 2, in a case where a steady-state displacement is given to the spring part made of a copper alloy sheet and the male terminal (tab) 6 is mated with the contact point having a spring shape (push part) 5, and when kept under a high-temperature environment like in an engine room, then it comes to lose its mating force at the contact point with the lapse of time. Accordingly, resistance property of stress relaxation means the property of resistance to high temperatures of those connecting parts of such that, even when kept under high-temperature environments, the mating force at the contact point of the spring part made of a copper alloy sheet is not greatly lowered.
FIGS. 1(a) and (b) show a test apparatus for resistance property of stress relaxation under this standard. Using the test apparatus, one end of a test piece 1 cut as a strip is fixed to a rigid test bench 2 and the other end is cantilevered and consequently bent (degree of bending: d), and after left as such at a given temperature for a given period of time, this is gradually unloaded at room temperature, and the degree of bending after unloading (permanent distortion) is calculated as δ. The stress relaxation ratio (RS) is expressed as RS=(δ/d)×100.
As copper alloys excellent in resistance property of stress relaxation, heretofore Cu—Ni—Si alloys, Cu—Ti alloys, Cu—Be alloys or the like are widely known. However, recently, Cu—Ni—Sn—P alloys have become used in which the amount of the additive elements is relatively small. The Cu—Ni—Sn—P alloys may form ingots in a shaft furnace which is a large-scale melting furnace having a wide opening to air, and the high productivity thereof enables great cost reduction.
Various proposals for methods for enhancing the resistance property of stress relaxation of those Cu—Ni—Sn—P alloys themselves have heretofore been made. For example, Patent Documents 1 and 2 mentioned below disclose that an intermetallic compound containing Ni and P in a matrix of Cu—Ni—Sn—P alloy is finely dispersed to thereby increase the electric conductivity and simultaneously enhance the resistance property of stress relaxation.
Patent Documents 2 and 3 mentioned below disclose that P content of a Cu—Ni—Sn—P alloy is reduced to give a solute copper alloy where precipitation of the compound containing Ni and P is reduced. Further, Patent Document 4 mentioned below discloses that the substantial temperature and retention time in finish heat treatment or annealing in production of a Cu—Ni—Sn—P alloy sheet is defined to thereby increase the electric conductivity and simultaneously enhance the resistance property of stress relaxation.
Further, in Patent Document 5 mentioned below, a fine compound containing Ni and having a size of 0.1 μm or less, as measured in an extraction residue method with a filter having an opening size of 0.1 μm, is increased in a Cu—Ni—Sn—P alloy while a coarse compound containing Ni and having a large size of more than 0.1 μM is reduced therein to thereby enhance the resistance property of stress relaxation in the direction perpendicular to the rolling direction. More concretely, the proportion of the coarse compound containing Ni and having a large size of more than 0.1 to the Ni content in the copper alloy is made to be 40% or less and the fine compound containing Ni and having a size of 0.1 μm or less therein is thereby increased.                Patent Document 1: Japanese Patent No. 2844120        Patent Document 2: Japanese Patent No. 3871064        Patent Document 3: JP-A-11-293367        Patent Document 4: JP-A-2002-294368        Patent Document 5: JP-A-2007-107087        